Thermionic dispenser cathodes have previously been fabricated by using a process of pressing and sintering metal powder so as to produce a material having a random distribution of holes or pores which are subsequently impregnated with active cathode materials, e.g., compounds of alkaline earth metals well-known in the art, or, a reservoir of these compounds may be positioned behind the sintered matrix. During operation, activating materials, such as Ba/BaO, are generated and migrate through the sintered metal pores out onto the emitting surface at operating temperatures. As is disclosed in U.S. Pat. No. 4,101,800, Thomas et al., the dimensions of the individual pores and the pattern of the pore arrays can exert a sizable influence over the potential performance of the cathode. Hence, it is desirable for the dispenser cathode to be a controlled porosity dispenser (CPD) cathode.
Current techniques available for making a uniform array of holes in the surface layer of the controlled porosity dispenser cathode (CPD) are not able to fabricate a porous structure at the optimum theoretical design. While successful CPD cathodes with Ba/BaO impregnants have been fabricated, theoretical calculations have determined that closer hole spacings and smaller hole diameters are required in order to optimize the emission properties and resultant life expectancy of the cathode. Current techniques, however, are limited to a minimum pore width of about 25 microns, and pore spacings of the order to 35 microns. These dimension are limited due to the fact that the minimum hole diameter which can be etched using conventional techniques is approximately equal to the thickness of the sheet being etched through. Since a minimum covering foil thickness of about 25 microns is required for sufficient mechanical strength and reliability, the smallest hole diameters and spacings thus far that have been achieved in practical CPD cathode surfaces are 25 micron holes on 35 micron spacings. In contrast, theoretical studies have shown that optimum emission characteristics are obtained when pore widths of 1-10 microns, pore spacings of 5-50 microns, and CPD sheet thicknesses of 25-100 microns are employed.